This invention relates to global position system (GPS) receivers and more particularly to a GPS receiver having computerized estimation and tracking of code phase, carrier phase/frequency and signal amplitude for hardware simplification.
The GPS is a navigation system which comprises a plurality of space vehicles (satellites) moving in space and transmitting radio frequency signals to earth. The spacer vehicles' locations at any selected time are precisely known. Thus, by measuring the timing relationship between received signals from different space vehicles, differences in receive timing from four or more space vehicles to the receiver can be used to determine the location of the receiver.
Space vehicle identification is made possible by modulating its carrier frequency signals (L1 and L2) with a P (precision) code and/or a C/A (coarse acquisition) code that is unique to that vehicle.
In the past, GPS receivers have used separate receiver channels for processing the high (L1) frequency signals and the low (L2) frequency signals transmitted by each satellite, and either separate receiver channels for each satellite of the GPS or the sequential operation of one receiver to receive the high and low frequency signals of each space vehicle for satellite tracking.
Then, GPS multiplexed receivers were introduced which utilized a single receiver channel under baseband processor control: first, to multiplex the L1 and L2 signals through the single receiver channel; and secondly, to multiplex amongst several satellites for tracking multiple space vehicles without reacquiring each space vehicle. In these systems the receiver baseband design was digital; thus, many receiver functions were implemented in software; the baseband software functions were also implemented on either hardwire or firmware. Those persons skilled in the art desiring more information concerning these GPS receivers are referred, respectively, to U.S. Pat. No. 4,485,383, issued Nov. 27, 1984 to Robert A. Maher and to U.S. Pat. No. 4,468,793, issued Aug. 28, 1984 to Charles R. Johnson et al.
In addition, an experimental GPS receiver/digital processing system has been operated. The basic technical approach of this receiver consists of a broadband, fix-tuned RF converter followed by a digitizer, digital-matched-filter acquisition section; phase- and delay-lock tracking via baseband digital correlation; software acquisition logic and loop filter implementation; and all-digital implementation of the feedback numerically controlled oscillators (NCOs) and code generator. Baseband in-phase (I) and quadrature phase (Q) tracking is performed by an arctangent angle detector followed by a phase-unwrapping algorithm that eliminates false locks induced by sampling and data bit transitions, and yields a wide pull-in frequency range approaching one-fourth of the loop iteration frequency. Those persons skilled in the art desiring more information concerning this receiver are referred to Ould and VanWechel, "All-Digital GPS Receiver Mechanization", Navigation: Journal of The Institute of Navigation, Vol. 28, No. 3, at 178, Fall 1981.
In all the above mentioned prior art devices, the clock signal for driving the PN code generator is provided by a code numerically controlled oscillator (NCO) implemented in hardware and the doppler corrected frequencies for phase tracking or frequency tracking the GPS carrier signal is provided by a carrier NCO implemented in hardware. The use of hardware NCOs unduly complicates the recevier's hardware, and increases the receiver's size, cost and efficiency.
In all of the prior art, a maximum of two samples (early and late) are used to form the estimate of code phase.